in apple dev center i have seen:
When you report progress to Game Center, two things happen:

If the achievement was previously hidden, it is revealed to the player. The achievement is revealed even if your player has made no actual progress on the achievement (a percentage of 0.0).
so...at the end of the game i report achievement to gamecenter, then the player clicks on achievement button, and he could see the hidden achievement even if it is not reached...???
how can i mantain hidden this achievement?